The causes of climate change

Why, then, does the climate change? And what is its time scale? There is no short, complete, or even adequate answer to either question, and most of the ideas which have been put forward remain controversial. For the sake of clarity climatic events can be put into three interlocking categories: those arising from events outside the earth; those generated within the terrestrial system; and those caused by man himself. All can deeply affect our lives; and each, however long-term and apparently farfetched, deserves consideration.

In the last resort, all major change must come as a result of the earth's relationship with the sun, that nuclear power station 93 million miles away. Any interference with the energy supply across space, any change in the way that energy is received on earth, and any change in the supply system itself would clearly have immediate and substantial effects. Here, then, is the first place to look for the causes of the major changes represented by the successive ice ages during the last 1,000 million years.

Unfortunately the results of current work are ambiguous and inconclusive. One idea suggested by McCrea [2] is that as the solar system rotates as part of a general rotation of our galaxy, it encounters areas of dust in the spiral arms, and this dust affects the solar radiation reaching the earth. According to McCrea, the earth is now emerging from the spiral arm in Orion, and we are set fair for warmer conditions.

More violent but probably temporary changes in climate could be caused by the impact of such extraterrestrial objects as comets, planetoids, meteorites large and small, or other debris from space. In the history of the earth there have been sudden extinctions of species. The most notorious came at the end of Cretaceous times some 65 million years ago, when the dinosaurs ended their long career of dominance.

Whether this was a result of a collision with some extraterrestrial objects is uncertain. But the mechanism for extinctions of this kind could well have been climatic: darkening of the earth's atmosphere with dust from its surface, leading to a prolonged arctic winter in which photosynthesis and the food chain were disrupted. This event could be periodic. It has been suggested that the sun has a companion star which comes close enough to the solar system every 26 million years or so to perturb the orbits of the millions of comets in its neighbourhood. The nearest analogy to the result for us would be a major nuclear war.

A lesser event in the form of showers of micro-meteorites could have an effect similar to major volcanic eruptions on the earth's surface: the creation of a curtain of dust and ice crystals high in the stratosphere which, by intercepting and reflecting back into space the sun's radiation, could cause rapid chill below, and conceivably set the conditions for an ice age.

Moving from millions to thousands of years, and from interference with the energy supply to its receipt on earth, we come to the three variations in the earth's position with respect to the sun, known collectively as the Milankovitch effect (although the observation was first made by James Croll in 1864).

The amount of solar radiation reaching the earth varies according to the slight eccentricity of the earth's orbit, sometimes almost circular, sometimes elliptical, in an oscillation of between 90,000 and 100,000 years. When the orbit is most elliptical, the intensity of solar radiation can vary by up to 30 percent in the course of a year (at present it is around 7 percent).

Another variation arises from the shifting tilt of the earth's axis of rotation in relation to the plane of its orbit in an oscillation of 40,000 years. The greater the tilt, the more the difference between summer and winter (the last maximum was 10,000 years ago).

Finally there is a variation, this time of 21,000 years, in the wobble of the earth around its axis - the so-called precession of the equinoxes - and this determines the time at which the northern or southern hemisphere is closest to the sun (at present it is the largely oceanic southern hemisphere).

Much work has been done to assess the effects of these three major variations, and although few would claim that the relationship between them could alone cause the alternation between glacial and warm conditions in the last one and one-half million years, (if it were the sole cause, this oscillation would be permanent rather than - as is the case - exceptional), most are agreed that it has been a major factor.

Tables have been made which show that the predicted variations in sunshine reaching a given point on the earth's surface during the last 700,000 years can be correlated with the actual changes of temperature recorded in isotopic analysis of certain marine fossils. Certainly we must reckon with the Milankovitch effect in calculating our longer term future. [3]

Unless other factors intervene, the world could face renewed glaciation around 4,000 years from now; then a mild improvement; then a deeper plunge into glaciation around 23,000 years from now; again improvement, followed by a return to conditions similar to the worst rigors of the last ice age some 60,000 years in the future. [4]

Of more importance in the short term is the possibility of variation in solar radiation itself. Acceptance of the idea that the emission of energy from the sun is not perfectly uniform, and that the so-called solar constant might turn out not to be so, is relatively new. The sun is subject to cyclical changes in behaviour, ranging from a pulsation rate of around 2 hours 40 minutes, rotation on its own axis every 27 days, sunspot activity over 22 years, to much longer fluctuations over thousands or perhaps millions of years.

The connection between sunspots and the weather on earth is part of universal folklore and cannot easily be dismissed, although the means by which one could affect the other has not been convincingly demonstrated. Was it a coincidence that between 1645 and 1715, the worst of the Little Ice Age, sunspots became a fascinating rarity to astronomers; the display of solar activity in the upper atmosphere known as aurora (borealis or australis) was greatly reduced; and the dramatic jets and streamers, or coronae, usually seen around the sun at the time of an eclipse, went unreported?

Sunspots are, of course, no more than a sign of magnetic activity in the sun, but their presence or absence could indicate variations in radiation. Lack of sunspots in 1976 - the low point in the 22-year cycle - was blamed for droughts in temperate parts of the northern hemisphere. Whatever may be happening now, it has been estimated that during the period 1645 to 1715, radiation could have been around 1.0 percent lower than at present, a figure not inconsistent with what we can assess were average prevailing temperatures in the northern hemisphere at that time (about 1ºC to 2ºC less than at present). [5]

Our knowledge of the behaviour of the sun has, anyway, been shaken by the failure over three years of experiments, to find the predicted numbers of the supposedly massless particles known as neutrinos in the spectrum of solar radiation. Either the experiments were in some way defective or our thinking about neutrinos (which might after all have some infinitesimal mass) and the nuclear processes at work in the sun, is wrong or incomplete.

One suggestion is that imbalances develop over millions of years between the heat at the centre and at the surface, and that the solar engine, whose heat at the centre could be less than calculated, is returning to equilibrium after an anomaly, which could be associated with the last ice ages.

A high priority is more precise measurement (to within 0.1 percent accuracy) of solar radiation over a period of years. Another is to determine the effects on such radiation of variations in the earth's magnetic field. Without such information we can do little more than speculate about what could be crucial factors in variations of the earth's climate.

Two other external factors should be mentioned:

• The differential rotation of the planets, including the earth, has tidal effects on the surface of the sun, and the disturbances thus created could affect the earth at a rhythm of about 1,700 years.
• The pull of sun and moon, which creates the familiar ocean tides, has other effects on the surface of the earth:
  • Volcanic eruptions, throwing up cooling dust veils into the atmosphere, tend to occur in response to the maximum combined forces of sun and moon, perhaps in a periodicity of 200 years.
  • As for the oceans, exceptionally high tides at intervals of about 1,800 years, can change weather patterns by setting processes in train which take long to correct themselves: for example, the tidal maximum in high latitudes in 1433 may have contributed to the southward drift of Arctic pack ice, which in turn contributed to the Little Ice Age. [6]

There are other lesser factors. But those cited are enough to show the total dependence of the earth and its climate on events in the sun and solar system generally. As has been well said, men have worshipped things more foolish than the sun.

Of natural changes to the climate arising from within the terrestrial system, by far the most important are caused by the slow movement of the various pieces of the earth's crust - the so-called tectonic plates - in relation to each other. The drift of continents, the rise and erosion of mountains along the plate boundaries, and the distribution of the seas, determine how the energy arriving from the sun is distributed within the earth's atmosphere.

The discovery of fossilised tropical plants in Antarctica, of scratches left by glaciers in Brazil, or of seashells high in the Alps is less amazing when we think of the earth's crust as a cracked and bulging plastic cover, subject to varying pressures from inside. Changes up, down, or sideways of a few centimetres a year may not seem relevant to modern problems, or at most of interest only to scientists. But they are crucial to our understanding of why the climate should be as it is, and provide an important part of the explanation for the most recent as well as earlier ice ages.

The presence of land or landlocked sea at the poles seems indispensable for the growth of the glaciers which make up the present ice caps. Perennial ice now covers 11 percent of the earth's land surface, and 7 percent of its seas. The drift of the Antarctic continent to its present isolated position, and the subsequent partial enclosure of the Arctic Ocean from most of the warm currents moving north from the equator, helped the slow accretion of ice sheets.

Without these permanent glaciers (some in Antarctica are probably more than 20 million years old), the processes which led to the successive advances and retreats of the ice in the last one and one-half million years could probably have never got started. Thus a particular configuration of land, sea, and ice is the stage on which all climate is set. If the Gulf Stream were to alter its north-easterly flow a little to the south, as it did 18,000 years ago, the British Isles and Western Europe could again have the climate of the Hudson Bay in Canada on the same line of latitude (see Figure 4).

Figure 4: Positions of the boundary in the ocean surface between water of Gulf Stream origin and the polar ocean current from near northeast Greenland in the 20th century warmest years and at various times past.
Courtesy of H. H. Lamb and Methuen Ltd.

The ice caps themselves occupy a vital role in the world weather system, profoundly affecting ocean currents and winds, and the transfer of heat between them. The Greenland and the East Antarctic Sheets, both grounded on land above sea level, are its anchors. By contrast, most of the Arctic Sheet is unstable and sensitive to relatively small changes in air and ocean temperature. The West Antarctic Sheet, whose base is below sea level, may also be unstable.

Ice can generate its own climate in the process known as positive feedback. Once established - and that can be fast or slow - snow and ice reflect the bulk of the sun's radiation back into space. This creates colder weather, which in turn leads to the extension of the snow and ice cover. This self-reinforcing process can eventually reach stability, correct itself, or go into reverse, when lack of evaporation from the snow and ice over a large area reduces the quantity of precipitation in the form of new snow. Wind-blown dust from elsewhere can also reduce the reflectivity (or albedo) of the snow surface, and affect the process described above.

Several theories to explain the apparent rhythm of ice ages have been built on the interaction of ice and ocean current and wind. Most agree that there is an imbalance in the energy budget of ocean and atmosphere, which causes an oscillation over tens of thousands of years between a warm mode and a cold mode. For present purposes it is enough to say that each is unstable.

In the warm mode the shrinking of the ice caps brings warm water further towards the poles and a rise in sea level. Evaporation increases and with it the amount of precipitation. At the same time water cooled at the poles moves towards the equator. But increased snow at the poles and cooler water in temperate latitudes switches the world over to the cold mode. The ice sheets grow, sea levels drop, and temperatures fall in middle latitudes.

Eventually this too corrects itself. The warm mode is resumed when the supply of snow fuelling the ice caps falls with decreased evaporation, and when the surface water, warmed by the sun, is no longer sufficiently cooled by contact with the ice (itself now extended into warmer latitudes) to have cooling effects elsewhere.

Another idea is that periodically vast quantities of West or even East Antarctic ice become unstable and surge out to sea, raising sea levels, lowering temperatures further north, increasing storminess, and eventually causing greater precipitation at the poles. A rapid rise in sea level of 5 - 7 meters 125,000 years ago, accompanied by oceanic cooling of a few degrees Celsius, could have been such an event. In any case, the state of the polar ice sheets and the drift of ice north and south from them is a critical factor in determining the weather, as well as the climate throughout the world.

The process of positive feedback does not apply only to ice. Deserts also reflect back more heat than they receive, and can, if recent evidence is correct, both draw in and chill the air over them. Precipitation from outside, such as it is, is reduced, and the desert can thus extend itself.

Other examples abound. One is the way in which a change in the temperature of the ocean surface can alter the quantity of heat absorbed by the air, which in turn causes changes in the atmosphere, which in turn enhances the change in the temperature of the ocean. The importance of the mechanism is that it gives relatively fast and prolonged momentum to what might otherwise seem slow and minor elements in climatic change. Even some ephemeral, including human, activity could press a button and start a process which might not correct itself for a long time.

In this respect much recent work has been done on the role of volcanic emissions in the atmosphere. Volcanoes, most of which are along the boundaries of tectonic plates, blast immense quantities of matter into the sky during eruptions. Some of this reaches the stratosphere where it forms a veil of tiny particles (including droplets of sulphuric acid) around which ice can form. Such particles can be compared in their effects with micro-meteorites.

Much depends on the latitude of the eruption. Those within 20º of the equator cause a veil to be spread over the whole earth as the particles move towards the poles where they last longest. Eruptions in higher latitudes tend to be limited to the hemisphere in which they take place. Depending on the size, colour, and shape of the particles, they shut out more solar radiation from outside than terrestrial radiation from inside. The broad result is to cool the surface of the earth.

According to observations made at Mauna Loa in Hawaii, the eruption of Mount Agung in Bali in 1963 led to a decrease of nearly 2 percent in direct solar radiation and of around 0.5ºC in average world temperatures. The eruption of El Chichonal in Mexico in 1981 had a comparable result. In spite of some countervailing effects, eruptions seem to be accompanied by stronger and wetter wind circulation in temperate latitudes.

Some, if not many, of the coldest wettest summers in Western Europe and eastern North America during the last 300 years may have been so caused. A case that has been minutely examined, is the summer of 1816 following the eruption of Mount Tambora in what is now Indonesia the year before. [7] On average, around a million tons of dust from all sources, including deserts, fall to the earth's surface every year.

This figure can be compared with up to a 100 million tons which can fall after a major series of eruptions. Thus could be initiated a process lasting much longer than the two to seven years which follow a single eruption. Indeed it could be one of those buttons, which, if pressed could put the weather machine into a different mode. Evidence from an ice core taken from the Byrd glacier in Antarctica, shows a steep increase in deposits of micro-particles during the coldest years of the last ice age between 18,000 and 22,000 years ago.

It also shows during the ice age a steady natural decline in atmospheric carbon dioxide down to around 200 parts per million (ppm). This may be compared with the present partly man-made rise to around 340 ppm, which has led to fears of an overheated greenhouse earth (of which more below) (see Figure 5).

Figure 5: Findings from Antarctice ice cores, showing the relationship between oxygen isotopes, microparticles, atmospheric carbon dioxide, and temperatures over the last 40,000 years.
Courtesy of the British Antarctic Survey.

More is known of effects than causes of the rhythms of wind and sea in inducing climatic change. In the northern hemisphere, some winds behave with fair regularity within the limits of the present system, but the circumpolar jet stream or vortex, blowing from west to east about 6 miles above the surface, is virtually a law unto itself. On its speed and strength, and on its zigzags over Europe, the Soviet Union, and North America, depend much of the weather in northern temperate latitudes and thus the harvests in the main grain-growing areas of the world.

If the jet stream moves north, it can suck up warm wet air after it from the south; if it moves south it can bring in cold dry air from the Arctic. No wonder that its swings and eddies are so carefully watched, particularly in the Soviet Union. The same goes for variations in the Asiatic monsoon. Another much slower natural variable is the apparent tendency of waves of weather to move westward at an almost constant rate of 0.60 of longitude a year, thus making the circuit of the world in just under six centuries.

The variables in the sea are still less understood. We have already looked at the possibility of long oscillations creating and destroying parts of the polar ice caps. Another possible factor is variation in the earth's magnetic field: there is some correspondence on both long and short time-scales between weakening of the magnetic field and warmer ocean temperatures.

More than half the solar radiation reaching the surface of the earth is absorbed by the sea, largely in the top 300 feet. This acts as a giant reservoir of heat: some is evaporated into the atmosphere, some is moved and mixed downwards, and some is kept in the surface layer and travels under the influence of the wind, differences in temperature and saltiness, and the rotation of the earth itself.

The movement of currents and of huge slow eddies in the ocean (akin to storms in the atmosphere) have rhythms on time-scales ranging from the length of a season to tens of thousands of years. They are of intimidating complexity but their influence on climate can be decisive.

One example will suffice. In normal circumstances a strong Pacific current runs from the south northwards along and away from the coast of Peru. This creates an upwelling of deep, cold water rich in nutrient salts, which nourishes the fish, on which much of the Peruvian economy depends, and determines the local weather.

Every now and again, roughly once in two to ten years, the current moves down and offshore (a phenomenon known as El Niño, or the Child, for its frequent appearance around Christmas time). Warm water moves in from the north with catastrophic results for men and fish alike. The immediate reason for these events seems to be an interaction between a strengthening of the trade winds in the western tropical Pacific and abnormally high ocean temperatures in the eastern tropical Pacific.

Recent research following the last visit of El Niño in 1982 has shown that El Niño is linked to a wider anomaly called the Southern Oscillation which disturbs weather patterns over the whole Pacific. As the temperature of surface Pacific waters influences the performance of the circumpolar jet streams, the pattern of climate in the temperate parts of the northern as well as the southern hemispheres is also affected. For a few months the world's weather is out of joint. Then gradually it reverts to familiar patterns. No one has yet determined the underlying causes. World weather is poised on a delicate balance. To upset one factor is to risk upsetting the lot. [8]

With so many natural variables, each interacting with the other, how can we hope to understand, let alone predict, the behaviour of the system as a whole? Most scientific work is done by reducing problems to manageable size in laboratories, but our only laboratory for climatic experiments is the earth itself.

Although we have been able to observe and measure the effects of prolonged dust storms on the planet Mars and those of concentrations of carbon dioxide on the planet Venus, we have no means of trying out the effects of particular climatic events - even if it were feasible - without ourselves being involved in, or rather subjected to, the consequences. This has particular importance when we turn to what man himself may now be doing to the climate.

We now enter an area of even greater uncertainty. On one point most people are agreed. By itself no human activity has - yet - altered or substantially affected the climate of the world as a whole. That is not to say that in combination with other factors human beings have not already contributed to, or possibly mitigated, change. Nor that their activities have not had local or regional effects. Nor that before very long they could not set processes in train which could lead to minor fluctuations or conceivably major change.

In some respects they may already be straining the tolerance of a system whose variable parts do not, as we have seen, need much to upset them. These variables comprise most of the elements in the heat balance: the myriad ways in which the earth's surface absorbs or reflects radiation from the sun; the amount of heat otherwise generated at the surface; and the contents of the earth's atmosphere with its load of dust, particles, and gaseous constituents (including the ozone layer in the stratosphere).

Human beings and their domestic animals have been gnawing away at the surface of the earth for a long time. Something like 20 percent of the total area of the continents has been drastically changed. The cutting down of trees for settlement and agriculture, the slash-and-burn method of cultivation in primitive societies, the proliferation of such grazing animals as goats and cattle, and the overuse and impoverishment of top soils have in the past affected the heat and water balance in specific areas.

The amount of solar radiation absorbed by the grass, crop, or in some cases desert land which took the place of the ancient forests was less than before. More was reflected back into space, less moisture evaporated as rain, and more rain - when it came - was run off. The inevitable increase of dust blowing up from the surface may have had local effects similar to those of volcanic eruptions or sandstorms.

For human beings the most important results of such changes were in the pattern of rainfall, and to a lesser extent temperature. For example, the progressive aridity in historical times of the swathe of land from the Mediterranean to northern India, once covered in dense forest and later the site of successive civilisations, seems to have been caused mostly by human destruction of the natural environment.

Certainly most of the grain-growing areas which supported large populations, including those of the Roman Empire, are now largely scrub or desert. According to recent calculations, around 7 percent of the earth's surface - an area larger than Brazil - is man-made desert. On the worst hypothesis the current desert area of around eight million square kilometres could triple by the end of the century. Even if the main effects remain local or regional, they will nonetheless contribute as in the past to changing climate.

In recent times two other man-made changes to the surface of the earth have become significant. The building of cities may not have affected the reflectivity of the earth as much as the substitution of grass, crop, or desert land for forest; but taking account of concrete buildings, the canyons between them, and roads (roads now cover almost 1.0 percent of the United States), and still more of the artificial energy generated within them, cities emit a good deal more heat than they receive: New York about six times more, Moscow three, and Sheffield one and one-half.

They are notably warmer than the surrounding countryside (between 10C to 30C more in winter), and cast a kind of heat shadow according to the direction of the prevailing wind. The global quantity of energy generated by mankind is only around .01 percent of that poured on top of the atmosphere by the sun. But this is of course increasing fast, and might one day be significant.

In the meantime existing heat domes over urban areas undoubtedly change local climates. The warm air rises, increasing cloud and turbulence, which in turn increases rain and snow fall. But rain and snow rapidly disappear and surface humidity is relatively low. The results are complex and analysis of them is incomplete. But they range from the suggestion that Atlantic winter storms are attracted towards the heat dome over the urban concentration known in the trade as Bosnywash (Boston-New York-Washington) to the discovery that there is less rain and snow in cities (and downwind of them) at weekends when factory smoke and car exhausts are diminished. [9]

The second recent man-made change is the creation of artificial lakes and extended areas of irrigation, and the diversion of river systems. Irrigation is not of course new, and may have had minor climatic effects in the past (for example, the waterlogging and salinisation of much of Mesopotamia contributed to its deterioration into desert). But the vast lakes and irrigation schemes of our own time are of a new order of magnitude: depending on the angle of the sun's rays, water retains and absorbs more heat than any land surface, and through evaporation adds vapour to the atmosphere. As a general rule, this should produce more cloud and higher rainfall downwind, but each case is of course particular to itself.

The diversion of rivers can have even greater effects. A good example - and disturbing case - is the proposal to divert some of the water from the northward flowing rivers of Siberia to the arid lands of Central Asia. Whatever the beneficial effects in Central Asia, the loss of fresh water in the Arctic could in sufficient quantity affect the polar weather pattern: the salty sea water would freeze less readily, evaporation might increase, causing more snowfall elsewhere, and existing currents, including the inflow of Atlantic water, might change.

All this could affect the balance of permanent glacier and Arctic pack ice, and eventually modify the existing heat exchange between ocean and atmosphere, and of course the wind system in the northern hemisphere as a whole. Like such ideas as damming the Bering Straits or altering the flow of some of the Gulf Stream, the proposal to divert water from the Siberian rivers raises major international issues. It has already caused much anxious debate, not least in the Soviet Union itself (see Figure 6).

Figure 6: Scheme for the reversal of the flow of the Russian and Siberian rivers, the Pechora, Ob and Yenesei, to water the arid lands in Soviet Central Asia.
Courtesy of H. H. Lamb and Methuen Ltd.

Perhaps the most obvious man-made change to the climate follows not from what people do to the surface of the earth but from what they deliberately put into the sky, an even bigger waste disposal unit than the sea. Since the beginning of the industrial revolution, the combustion of fossil fuels - coal, oil, and gas - has led to a steady increase in the quantity of carbon dioxide gas (C02) and aerosol particles in the atmosphere.

Half or more of the increased supply of carbon dioxide finds its way into the sea or is absorbed by plants, but the remainder lingers in the air. No one knows what the 'natural' quantity of carbon dioxide was in pre-industrial times. It has varied greatly over the history of the earth for reasons so far uncertain and, as we have seen, could have been as low as 200 ppm during the last ice age.

By the middle of the last century it was probably over 270 ppm, increasing to 290 ppm between 1880 and 1890. Thereafter its rise has continued to be steep: up to 314 ppm in 1958, when the first reliable measurements were made, to over 340 ppm in 1983, and still rising fast. The increase is almost certainly due to the accelerating consumption of fossil fuels and the destruction of forests (which reduces the accumulation of carbon dioxide through photosynthesis and increases its release through decomposition).

The primary effect of increasing carbon dioxide in the atmosphere is to change the equilibrium between incoming radiation from the sun and outgoing radiation from the earth. By blocking some of the infrared radiation from the earth and radiating it back to the earth's surface, it warms the lower atmosphere and cools the upper atmosphere. This in turn increases moisture, which traps more infrared radiation. The effect of rising temperature is to reduce the areas covered by snow and ice, thereby diminishing the amount of heat reflected back into space, and increasing absorption of solar radiation. The precise effect of greater cloudiness is uncertain: more high thin cloud would tend to retain heat, and more low thick cloud to reflect it back into space. [10]

The weighing up of these factors is not easy. The models which have been used are inadequate, and the information fed into them is incomplete. Other factors are operating, not least the effect of other trace gases, particularly methanes and nitrous oxide, which some believe could add substantially to the carbon dioxide effect. Whether, and if so by how much, there has been any warming is in dispute. Some calculations show that the temperature of the lower atmosphere should already have warmed by between 0.2ºC and 0.3ºC. But there is no proof of that so far, and other factors may have been operating in the other direction.

Carbon dioxide, methanes, and nitrous oxide are not the only chemicals we have discharged into the atmosphere. The quantity of man-made aerosol particles - industrial dust and smog - has also increased. Here the primary effects are local. Most particles, like other forms of dust, are rained back on the surface fairly soon, and do not spread far from the areas, usually the urban concentrations of the northern hemisphere, whence they came.

With more industrialisation their effects will obviously increase, but what those effects are is still in dispute. The extent to which they absorb or reflect solar and terrestrial radiation depends on their size, colour, and shape. Here even the best authorities can arrive at opposite conclusions. One believes that aerosol particles should recently have cooled the northern hemisphere by around 0.5ºC, thus counteracting the rise in temperature caused by carbon dioxide; while another does not rule out the possibility that they have reinforced the carbon dioxide effect and led to warming effects at the surface in the same area.

Still higher in the gaseous envelope surrounding the earth is a wavy layer containing a small but vital quantity of the oxygen molecule 03, or ozone (some .001 percent). Ozone is constantly created by the action of sunlight on normal oxygen molecules, and is as constantly destroyed after about 18 months through interaction with nitric oxide and other molecules rising from the earth. During that time it drifts from the equator towards the poles, where the ozone layer is at its thickest. It is subject to considerable natural fluctuation, and responds to changes in solar activity. Over the 22-year sunspot cycle it may vary by as much as 12 percent. So far relatively little is known about its behaviour. But without ozone, life in its present form literally could not exist.

Ozone has two closely related functions. First it acts as a buffer to much of the ultraviolet radiation from the sun (in particular the wavelength of 0.26 millionths of a meter which would otherwise damage the DNA or reproductive molecule in all living systems). As a rule of thumb, ultraviolet radiation reaching the ground increases about 2 percent for each 1 percent decrease in atmospheric ozone. Variations in the thickness of the ozone layer are reflected in the ability of organisms, including the human species, to cope with different degrees of ultraviolet radiation. Thus, white-skinned people who venture into the tropics or climb to high altitudes are more subject to sunburn and skin cancer than the brown - or black - skinned inhabitants.

Secondly, by absorbing ultraviolet radiation the ozone layer heats the stratosphere, causing the familiar problem of heat inversion as observed so often at a much lower level over such cities as Mexico and Los Angeles. Ozone thus forms a kind of lid on the atmosphere, and a rich variety of natural and man-made products gets trapped beneath it.

Some may do it active harm. The most notorious are the chlorofluorocarbons (known commercially by a variety of names, including freons), which are used as a propellant in spray or aerosol cans and as a refrigerant in cooling devices. Chlorofluorocarbons have a similar effect to carbon dioxide in blocking some infrared radiation from the earth, and thus retaining heat at its surface. They rise slowly into the stratosphere, where they are turned by the actions of sunlight into fluorine and chlorine atoms, some of which destroy ozone.

The chemical oxides of nitrogen are less efficient but some again have a similar effect: here the most important are those (NO x) injected into the stratosphere from the exhausts of high-flying aircraft, or from nuclear explosions, or from chemical processes on earth. Nor is this all. Other suspects are the nitrous oxide (N2O) produced in nitrogen fertiliser, brominated and chlorinated compounds used in the purification of drinking water and sewage, and carbon monoxide from car and other exhausts.

There is as yet little certitude about the effects of human activity on the ozone layer. With wide normal fluctuations, the human contribution is hard to determine, but some models suggest that so far it is less than 1 percent. But it is more what could happen than what has happened which is alarming. Continuing increase in the manufacture of chlorofluorocarbons over the years or, as many would like to see, the multiplication many times of current production of nitrogen fertiliser, might eventually cause significant depletion of the ozone layer.

When then? Most attention has so far been focused on a possible increase in the incidence of human skin cancer. But however unpleasant for the victims, this is less serious than the consequences for life as a whole. Experiments have shown that greatly increased exposure to ultraviolet radiation (more than would be likely from ozone depletion for a long time) slows up the growth of plants on land or in the sea, accelerates genetic mutation, is lethal to certain higher organisms, and damages the process of photosynthesis. Thus, the food chain itself might conceivably be affected. A catastrophe of this kind may already have happened more than once in the history of the earth. It has been suggested that substantial depletion of the ozone layer at the various times when the earth reversed its magnetic field could have contributed to the extinctions of species which have taken place from time to time in the past.

As for climate, the thickness of the ozone layer critically determines the temperature of the stratosphere. This in turn affects temperature at the surface. Here may be the mechanism by which variations in solar weather - from the great wind of particles which blows continuously in all directions to magnetic storms on the surface and corona - affect weather on the earth. We weaken our protection against solar turbulence at our peril.

By comparison with these inadvertent tamperings with the weather machine, deliberate human manipulation of weather conditions has so far been puny in its effects. For thousands if not millions of years rainmakers have tried to work their magic. At least in theory it is now possible to induce rain, prevent hail, disperse fog, frustrate hurricanes, and deluge enemies. But in practice the seeding of clouds with dry ice or silver iodide has had only local results, generally below expectations.

It is always difficult to demonstrate cause and effect, and seeding has its believers and non-believers. But the potentialities for this as for other forms of weather modification are obvious, and carry wide, including international, implications. One man's rain can be another man's drought. Even the dispersal of a typhoon, which saves one area from destruction, means denial of life-giving water to another.

The biggest bangs we can make are nuclear, and over the years nuclear tests have been under popular suspicion for changing the climate or causing almost any apparent abnormality. So far as is known, the climatic consequences of isolated explosions of this kind are not very different from those of volcanic eruptions. Whether the effects are local or general, short- or long-lived, depends on the size and altitude of the explosion, and the quantity of dust and other particles discharged into the atmosphere.

A major nuclear war would be another story. As suggested earlier, the nearest analogy might be the impact of some such extraterrestrial object as a comet. Recent enquiry has suggested that the use of less than 1 percent of available nuclear weapons on 100 to 200 cities could, by surrounding the earth with a thick cloud of dust and soot, plunge the world into darkness which would create a prolonged Arctic winter.

There are many uncertainties arising from the multiplicity of doubtful assumptions, as well brought out in a recent report by the U.S. National Academy of Sciences. But from a disaster of this magnitude, the survival of our species and other higher organisms could not be assured. Even when the dust eventually settled and the ozone layer was restored, the devastation of the earth's surface might increase its reflectivity to solar radiation, and the continuation of glacial conditions over part of it could not be excluded.

The long-term consequences of a disaster of this kind cannot be assessed; but the destruction of the environment could be even more devastating for life than the well-known effects of increases in radioactivity. [11]